1. Field of the Invention
The present invention relates to a high-frequency power amplifier, and more particularly to a high-frequency power amplifier for use in cellular telephones, etc.
2. Background Art
Conventionally, high-frequency power amplifiers such as RF power amplifiers have been used in the antenna output stage or the audio output stage of communications equipment.
High-frequency power amplifiers producing an output of 1 W (30 dBm) or so generally require an output matching circuit to supply the power to a 50 Ω load. FIG. 6 shows the structure of a module example formed by integrating such an output matching circuit with a high-efficiency high-frequency power amplifier (for example, see a paper entitled “High-Frequency 0.1 cc Power Amplifier Module for 900 MHz Personal Digital Cellular Telephones” by Akira Inoue et al., IEIC Trans. Electron, Vol. E82-C, No. 11, pp. 1906–1912, November 1999). In the example of FIG. 6, the matching circuit includes capacitors, inductors, signal lines, etc. To miniaturize the high-efficiency high-frequency power amplifier, chip components having the 1005 size (1.0 mm×0.5 mm) or the 0603 size (0.6 mm×0.3 mm) are used as the capacitors and inductors, which are mounted on a substrate formed of resin, glass ceramics, etc. In the figure, reference numeral 100 denotes a module substrate, 101 denotes a transistor, and 102 denotes chip components.
The above chip components in the high-frequency power amplifier are connected to one another by use of microstrip lines formed on the substrate or strip lines formed within the substrate. FIGS. 7A to 7C show a strip line used in the high-frequency power amplifier shown in FIG. 6. Specifically, FIG. 7A is a plan view of the strip line portion, wherein reference numeral 103 denotes the strip line.
FIG. 7B is a cross-sectional view of the strip line portion shown in FIG. 7A taken along line VIIB—VIIB. As shown in FIG. 7B, the strip line has a 3-metal-layer structure (or a structure including 3 metal layers) in which grounding conductors (hereinafter referred to as GNDs) 105 and 106 sandwich a strip conductor 104 within dielectric material.
The characteristic impedance of the strip line is expressed by Formula 1.(¼)×(μ/∈)1/2×(b/W),  [Formula 1]where μ denotes the magnetic permeability and ∈ denotes the dielectric permittivity. Further, the symbol b denotes the distance between the GND 105 and the GND 106, and W denotes the width of the strip conductor 104, that is, the signal line width, as shown in FIG. 7B. It is assumed that the distance of the GND 105 from the strip conductor 104 is equal to that of the GND 106 from the strip conductor 104.
Assume that there is a strip line having a characteristic impedance optimized through design of the output matching circuit, and a need has arisen to reduce the distance b between the GNDs without changing the characteristic impedance. In such a case, it is necessary to also reduce the signal line width W as indicated by Formula 1. Reducing the signal line width W, however, increases the DC resistance value of the line, resulting in increased loss produced in the strip line. Therefore, it is desirable to increase the distance b in terms of decreasing the loss in the strip line. It should be noted that an increase in the distance b means an increase in the thickness of the substrate constituting the high-frequency power amplifier.
On the other hand, the obverse and the reverse surfaces of the substrate of the high-frequency power amplifier may have wiring prohibited areas therein in which wiring for a strip line is prohibited. For example, in a chip component mounting portion of the substrate, the portion of the substrate top surface on which the non-GND terminals of the chip components exist cannot be used as a GND surface. Therefore, the region of the substrate from the chip component mounting portion to the GND surface within the substrate is set as a wiring prohibited area. FIG. 7C is a cross-sectional view of the strip line portion shown in FIG. 7A taken along line VIIC—VIIC. In the figure, reference numerals 107 and 108 denote wiring prohibited areas.
Further, a cavity structure as shown in FIG. 8 also requires a wiring prohibited area. Specifically, since transistors, etc. are mounted on a cavity portion 110 in a substrate 109, the cavity portion 110 is a wiring prohibited area.
Still further, as shown in FIG. 9, no GND can be formed on the portion of the reverse surface of the module on which a signal terminal portion 111 is disposed. On the other hand, a GND 112 can be formed on the portion of the rear surface on which the signal terminal portion 111 is not disposed. Therefore, the region of the substrate from the signal terminal portion 111 to a GND surface 113 is set as a wiring prohibited area.
Incidentally, the substrate has a laminated structure in which a plurality of dielectric layers are laminated. It often happens that wiring prohibited areas exist at the same position in the lamination direction of the substrate. FIG. 10 is a plan view of a wiring prohibited area portion. In the figure, the lamination direction of the substrate is perpendicular to the paper. A wiring prohibited area 115 established on the obverse side of the substrate overlaps wiring prohibited areas 116 and 117 established on the reverse side of the substrate. That is, the overlapped portions of these wiring prohibited areas exist at the same position in the lamination direction.
Of all available high-frequency power amplifiers, those exhibiting high efficiency are suitable for the transmitting units of communications equipment terminals such as digital cellular telephones. In recent years, efforts have been made to increasingly reduce the thickness of digital cellular telephones. As a result, there has been an increasing need for thinner and thinner high-frequency power amplifiers.
One way to make a high-frequency power amplifier thinner is to make its substrate thinner, which requires reducing the distances between the strip conductor and the GNDs of the strip line and the distance between the GNDs themselves (that is, the distance b in FIG. 7B). This, however, leads to the problem of increased loss generated in the strip line, as described above.
On the other hand, to increase the efficiency of the high-frequency power amplifier, the loss of the output matching circuit must be reduced. To do so, however, it is necessary to increase the distances between the strip conductor and the GNDs of the strip line and the distance between the GNDs themselves (that is, the distance b in FIG. 7B), which runs counter to reduction of the thickness of the substrate.
The present invention has been devised in view of the above problems. It is, therefore, an object of the present invention to provide a high-frequency power amplifier which is thin but can achieve high efficiency.
Other objects and advantages of the present invention will become apparent from the following description.